Published video coding standards include ITU-T H.261, ITU-T H.263, ISO/IEC MPEG-1, ISO/IEC MPEG-2, and ISO/IEC MPEG-4 Part 2. These standards are herein referred to as conventional video coding standards.
Video Communication Systems
Video communication systems can be divided into conversational and non-conversational systems. Conversational systems include video conferencing and video telephony. Examples of such systems include ITU-T Recommendations H.320, H.323, and H.324 that specify a video conferencing/telephony system operating in ISDN, IP, and PSTN networks respectively. Conversational systems are characterized by the intent to minimize the end-to-end delay (from audio-video capture to the far-end audio-video presentation) in order to improve the user experience.
Non-conversational systems include playback of stored content, such as Digital Versatile Disks (DVDs) or video files stored in a mass memory of a playback device, digital TV, and streaming. A short review of the most important standards in these technology areas is given below.
A dominant standard in digital video consumer electronics today is MPEG-2, which includes specifications for video compression, audio compression, storage, and transport. The storage and transport of coded video is based on the concept of an elementary stream. An elementary stream consists of coded data from a single source (e.g. video) plus ancillary data needed for synchronization, identification and characterization of the source information. An elementary stream is packetized into either constant-length or variable-length packets to form a Packetized Elementary Stream (PES). Each PES packet consists of a header followed by stream data called the payload. PES packets from various elementary streams are combined to form either a Program Stream (PS) or a Transport Stream (TS). PS is aimed at applications having negligible transmission errors, such as store-and-play type of applications. TS is aimed at applications that are susceptible of transmission errors. However, TS assumes that the network throughput is guaranteed to be constant.
There is a standardization effort going on in a Joint Video Team (JVT) of ITU-T and ISO/IEC. The work of JVT is based on an earlier standardization project in ITU-T called H.26L. The goal of the JVT standardization is to release the same standard text as ITU-T Recommendation H.264 and ISO/IEC International Standard 14496-10 (MPEG-4 Part 10). The draft standard is referred to as the JVT coding standard in this application, and the codec according to the draft standard is referred to as the JVT codec.
In the following, some terms relating to video information are defined for clarity. A frame contains an array of luma samples and two corresponding arrays of chroma samples. A frame consists of two fields, a top field and a bottom field. A field is an assembly of alternate rows of a frame. A picture is either a frame or a field. A coded picture is either a coded field or a coded frame. In the JVT coding standard, a coded picture consists of one or more slices. A slice consists of an integer number of macroblocks, and a decoded macroblock corresponds a 16×16 block of luma samples and two corresponding blocks of chroma samples. In the JVT coding standard, a slice is coded according to one of the following coding types: I (intra), P (predicted), B (bi-predictive), SI (switching intra), SP (switching predicted). A coded picture is allowed to contain slices of different types. All types of pictures can be used as reference pictures for P, B, and SP slices. The instantaneous decoder refresh (IDR) picture is a particular type of a coded picture including only slices with I or SI slice types. No subsequent picture can refer to pictures that are earlier than the IDR picture in decoding order. In some video coding standards, a coded video sequence is an entity containing all pictures in the bitstream before the end of a sequence mark. In the JVT coding standard, a coded video sequence is an entity containing all coded pictures from an IDR picture (inclusive) to the next IDR picture (exclusive) in decoding order. In other words, a coded video sequence according to the JVT coding standard corresponds to a closed group of pictures (GOP) according to MPEG-2 video.
The codec specification itself distinguishes conceptually between a video coding layer (VCL), and a network abstraction layer (NAL). The VCL contains the signal processing functionality of the codec, things such as transform, quantization, motion search/compensation, and the loop filter. It follows the general concept of most of today's video codecs, a macroblock-based coder that utilizes inter picture prediction with motion compensation, and transform coding of the residual signal. The output of the VCL are slices: a bit string that contains the macroblock data of an integer number of macroblocks, and the information of the slice header (containing the spatial address of the first macroblock in the slice, the initial quantization parameter, and similar). Macroblocks in slices are ordered in scan order unless a different macroblock allocation is specified, using the so-called Flexible Macroblock Ordering syntax. In-picture prediction is used only within a slice.
The NAL encapsulates the slice output of the VCL into Network Abstraction Layer Units (NALUs), which are suitable for the transmission over packet networks or the use in packet oriented multiplex environments. JVT's Annex B defines an encapsulation process to transmit such NALUs over byte-stream oriented networks.
The optional reference picture selection mode of H.263 and the NEWPRED coding tool of MPEG-4 Part 2 enable selection of the reference frame for motion compensation per each picture segment, e.g., per each slice in H.263. Furthermore, the optional Enhanced Reference Picture Selection mode of H.263 and the JVT coding standard enable selection of the reference frame for each macroblock separately.
Parameter Set Concept
One very fundamental design concept of the JVT codec is to generate self-contained packets, to make mechanisms such as the header duplication unnecessary. The way how this was achieved is to decouple information that is relevant to more than one slice from the media stream. This higher layer meta information should be sent reliably, asynchronously and in advance from the RTP packet stream that contains the slice packets. This information can also be sent in-band in such applications that do not have an out-of-band transport channel appropriate for the purpose. The combination of the higher level parameters is called a Parameter Set. The Parameter Set contains information such as picture size, display window, optional coding modes employed, macroblock allocation map, and others.
In order to be able to change picture parameters (such as the picture size), without having the need to transmit Parameter Set updates synchronously to the slice packet stream, the encoder and decoder can maintain a list of more than one Parameter Set. Each slice header contains a codeword that indicates the Parameter Set to be used. According to the JVT coding standard, there exist two kinds of parameter sets: one for the sequence (sequence parameter set) and one for the pictures (picture parameter set).
This mechanism allows to decouple the transmission of the Parameter Sets from the packet stream, and transmit them by external means, e.g. as a side effect of the capability exchange, or through a (reliable or unreliable) control protocol. It may even be possible that they get never transmitted but are fixed by an application design specification.
Transmission Order
In conventional video coding standards, the decoding order of pictures is the same as the display order except for B pictures. A block in a conventional B picture can be bi-directionally temporally predicted from two reference pictures, where one reference picture is temporally preceding and the other reference picture is temporally succeeding in display order. Only the latest reference picture in decoding order can succeed the B picture in display order (exception: interlaced coding in H.263 where both field pictures of a temporally subsequent reference frame can precede a B picture in decoding order). A conventional B picture cannot be used as a reference picture for temporal prediction, and therefore a conventional B picture can be disposed without affecting the decoding of any other pictures.
The JVT coding standard includes the following novel technical features compared to earlier standards:                The decoding order of pictures is decoupled from the display order. The frame number indicates decoding order and the picture order count indicates the display order.        Reference pictures for a block in a B picture can either be before or after the B picture in display order. Consequently, a B picture stands for a bi-predictive picture instead of a bi-directional picture.        Pictures that are not used as reference pictures are marked explicitly. A picture of any type (intra, inter, B, etc.) can either be a reference picture or a non-reference picture. (Thus, a B picture can be used as a reference picture for temporal prediction of other pictures.)        A picture can contain slices that are coded with a different coding type. In other words, a coded picture may consist of an intra-coded slice and a B-coded slice, for example.        
Decoupling of display order from decoding order can be beneficial from compression efficiency and error resiliency point of view.
An example of a prediction structure potentially improving compression efficiency is presented in FIG. 1. Boxes indicate pictures, capital letters within boxes indicate coding types, numbers within boxes are frame numbers according to the JVT coding standard, and arrows indicate prediction dependencies. Note that picture B17 is a reference picture for pictures B18. Compression efficiency is potentially improved compared to conventional coding, because the reference pictures for pictures B18 are temporally closer compared to conventional coding with PBBP or PBBBP coded picture patterns. Compression efficiency is potentially improved compared to conventional PBP coded picture pattern, because part of reference pictures are bi-directionally predicted.
FIG. 2 presents an example of the intra picture postponement method that can be used to improve error resiliency. Conventionally, an intra picture is coded immediately after a scene cut or as a response to an expired intra picture refresh period, for example. In the intra picture postponement method, an intra picture is not coded immediately after a need to code an intra picture arises, but rather a temporally subsequent picture is selected as an intra picture. Each picture between the coded intra picture and the conventional location of an intra picture is predicted from the next temporally subsequent picture. As FIG. 2 shows, the intra picture postponement method generates two independent inter picture prediction chains, whereas conventional coding algorithms produce a single inter picture chain. It is intuitively clear that the two-chain approach is more robust against erasure errors than the one-chain conventional approach. If one chain suffers from a packet loss, the other chain may still be correctly received. In conventional coding, a packet loss always causes error propagation to the rest of the inter picture prediction chain.
Two types of ordering and timing information have been conventionally associated with digital video: decoding and presentation order. A closer look at the related technology is taken below.
A decoding timestamp (DTS) indicates the time relative to a reference clock that a coded data unit is supposed to be decoded. If DTS is coded and transmitted, it serves for two purposes: First, if the decoding order of pictures differs from their output order, DTS indicates the decoding order explicitly. Second, DTS guarantees a certain pre-decoder buffering behavior provided that the reception rate is close to the transmission rate at any moment. In networks where the end-to-end latency varies, the second use of DTS plays no or little role. Instead, received data is decoded as fast as possible provided that there is room in the post-decoder buffer for uncompressed pictures.
Carriage of DTS depends on the communication system and video coding standard in use. In MPEG-2 Systems, DTS can optionally be transmitted as one item in the header of a PES packet. In the JVT coding standard, DTS can optionally be carried as a part of Supplemental Enhancement Information (SEI), and it is used in the operation of the optional Hypothetical Reference Decoder. In ISO Base Media File Format, DTS is dedicated its own box type, Decoding Time to Sample Box. In many systems, such as RTP-based streaming systems, DTS is not carried at all, because decoding order is assumed to be the same as transmission order and exact decoding time does not play an important role.
H.263 optional Annex U and Annex W.6.12 specify a picture number that is incremented by 1 relative to the previous reference picture in decoding order. In the JVT coding standard, the frame number coding element is specified similarly to the picture number of H.263. According to the JVT coding standard, frame number is reset to 0 at an IDR picture.
H.263 picture number can be used to recover the decoding order of reference pictures. Similarly, the JVT frame number can be used to recover the decoding order of frames between an IDR picture (inclusive) and the next IDR picture (exclusive) in decoding order. However, because the complementary reference field pairs (consecutive pictures coded as fields that are of different parity) share the same frame number, their decoding order cannot be reconstructed from the frame numbers.
The H.263 picture number or JVT frame number of a non-reference picture is specified to be equal to the picture or frame number of the previous reference picture in decoding order plus 1. If several non-reference pictures are consecutive in decoding order, they share the same picture or frame number. The picture or frame number of a non-reference picture is also the same as the picture or frame number of the following reference picture in decoding order. The decoding order of consecutive non-reference pictures can be recovered using the Temporal Reference (TR) coding element in H.263 or the Picture Order Count (POC) concept of the JVT coding standard.
The value of picture or frame number has a certain maximum. The next increment after the maximum value causes picture or frame number to be equal to 0. Typically, the value of picture or frame number is coded and transmitted as an N-bit unsigned integer, and consequently the maximum value of picture or frame number is equal to 2N−1.
A presentation timestamp (PTS) indicates the time relative to a reference clock when a picture is supposed to be displayed. A presentation timestamp is also called a display timestamp, output timestamp, and composition timestamp.
Carriage of PTS depends on the communication system and video coding standard in use. In MPEG-2 Systems, PTS can optionally be transmitted as one item in the header of a PES packet. In the JVT coding standard, PTS can optionally be carried as a part of Supplemental Enhancement Information (SEI), and it is used in the operation of the Hypothetical Reference Decoder. In ISO Base Media File Format, PTS is dedicated its own box type, Composition Time to Sample Box where the presentation timestamp is coded relative to the corresponding decoding timestamp. In RTP, the RTP timestamp in the RTP packet header corresponds to PTS.
Conventional video coding standards feature the Temporal Reference (TR) coding element that is similar to PTS in many aspects. The value of TR is formed by incrementing its value in the temporally-previous reference picture header by one plus the number of skipped or non-reference pictures at the picture clock frequency since the previously transmitted one. TR is typically coded as a fixed-length (N-bit) unsigned integer, and therefore modulo 2N arithmetic is used in calculations with TR. In some of the conventional coding standards, such as MPEG-2 video, TR is reset to zero at the beginning of a Group of Pictures (GOP).
In the JVT coding standard, there is no concept of time in the video coding layer. Picture order count is defined to be a variable having a value that increases with increasing picture position in output order relative to the previous IDR picture in decoding order or relative to the previous picture containing the memory management control operation that marks all reference pictures as “unused for reference”. Picture order count is derived for each frame and field. In the JVT coding standard picture order counts are used to determine initial picture orderings for reference pictures in the decoding of B slices, to represent picture order differences between frames or fields for motion vector derivation in temporal direct mode, for implicit mode weighted prediction in B slices, and for decoder output order conformance checking. Many encoders set picture order count proportional to sampling time of pictures.
Transmission of Multimedia Streams
A multimedia streaming system consists of a streaming server and a number of players, which access the server via a network. The network is typically packet-oriented and provides little or no means to guaranteed quality of service. The players fetch either pre-stored or live multimedia content from the server and play it back in real-time while the content is being downloaded. The type of communication can be either point-to-point or multicast. In point-to-point streaming, the server provides a separate connection for each player. In multicast streaming, the server transmits a single data stream to a number of players, and network elements duplicate the stream only if it is necessary.
When a player has established a connection to a server and requested for a multimedia stream, the server begins to transmit the desired stream. The player does not start playing the stream back immediately, but rather it typically buffers the incoming data for a few seconds. Herein, this buffering is referred to as initial buffering. Initial buffering helps to maintain pauseless playback, because, in case of occasional increased transmission delays or network throughput drops, the player can decode and play buffered data.
Transmission Errors
There are two main types of transmission errors, namely bit errors and packet errors. Bit errors are typically associated with a circuit-switched channel, such as a radio access network connection in mobile communications, and they are caused by imperfections of physical channels, such as radio interference. Such imperfections may result into bit inversions, bit insertions and bit deletions in transmitted data. Packet errors are typically caused by elements in packet-switched networks. For example, a packet router may become congested; i.e. it may get too many packets as input and cannot output them at the same rate. In this situation, its buffers overflow, and some packets get lost. Packet duplication and packet delivery in different order than transmitted are also possible but they are typically considered to be less common than packet losses. Packet errors may also be caused by the implementation of the used transport protocol stack. For example, some protocols use checksums that are calculated in the transmitter and encapsulated with source-coded data. If there is a bit inversion error in the data, the receiver cannot end up into the same checksum, and it may have to discard the received packet.
Second (2G) and third generation (3G) mobile networks, including GPRS, UMTS, and CDMA-2000, provide two basic types of radio link connections, acknowledged and non-acknowledged. An acknowledged connection is such that the integrity of a radio link frame is checked by the recipient (either the Mobile Station, MS, or the Base Station Subsystem, BSS), and, in case of a transmission error, a retransmission request is given to the other end of the radio link. Due to link layer retransmission, the originator has to buffer a radio link frame until a positive acknowledgement for the frame is received. In harsh radio conditions, this buffer may overflow and cause data loss. Nevertheless, it has been shown that it is beneficial to use the acknowledged radio link protocol mode for streaming services. A non-acknowledged connection is such that erroneous radio link frames are typically discarded.
Most video compression algorithms generate temporally predicted INTER or P pictures. As a result, a data loss in one picture causes visible degradation in the consequent pictures that are temporally predicted from the corrupted one. Video communication systems can either conceal the loss in displayed images or freeze the latest correct picture onto the screen until a frame which is independent from the corrupted frame is received.
In conventional video coding standards, the decoding order is coupled with the output order. In other words, the decoding order of I and P pictures is the same as their output order, and the decoding order of a B picture immediately follows the decoding order of the latter reference picture of the B picture in output order. Consequently, it is possible to recover the decoding order based on known output order. The output order is typically conveyed in the elementary video bitstream in the Temporal Reference (TR) field and also in the system multiplex layer, such as in the RTP header. Thus, in conventional video coding standards, the presented problem did not exist.
Buffering
Streaming clients typically have a receiver buffer that is capable of storing a relatively large amount of data. Initially, when a streaming session is established, a client does not start playing the stream back immediately, but rather it typically buffers the incoming data for a few seconds. This buffering helps to maintain continuous playback, because, in case of occasional increased transmission delays or network throughput drops, the client can decode and play buffered data. Otherwise, without initial buffering, the client has to freeze the display, stop decoding, and wait for incoming data. The buffering is also necessary for either automatic or selective retransmission in any protocol level. If any part of a picture is lost, a retransmission mechanism may be used to resend the lost data. If the retransmitted data is received before its scheduled decoding or playback time, the loss is perfectly recovered.